Pronounced electromigration of Cu in molten Sn-based solders
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Y.W. Lin and C.R. Kaoa) Department of Materials Science & Engineering, National Taiwan University, Taipei City 106, Taiwan (Received 25 July 2007; accepted 5 October 2007)
The high local temperature in flip-chip solder joints of microprocessors has raised concerns that the solder, a low melting temperature alloy, might locally liquefy and consequently cause failure of the microprocessors. This article reports a highly interesting electromigration behavior when the solder is in the molten state. A 6.3 × 103 A/cm2 electron current was applied to molten Sn3.5Ag solder at 255 °C through two Cu electrodes. The high current density caused rapid dissolution of the Cu cathode. The dissolved Cu atoms were driven by electrons to the anode side and precipitated out as a thick, and sometimes continuous, layer of Cu6Sn5. The applied current caused the dissolution rate of the Cu cathode to increase by one order of magnitude. A major difference between the electromigration in the solid and molten state was identified to be the presence of different countering fluxes in response to electromigration. For electromigration in the molten state, the back-stress flux, which was operative for electromigration in the solid state, was missing, and instead a countering flux due to the chemical potential gradient was present. An equation for the chemical potential gradient, d/dx, required to balance the electromigration flux was derived to be d/dx ⳱ N°z*eJ, where N° is Avogadro’s number, z* is the effective charge of Cu, e is the charge of an electron, is the resistivity of the solder, and J is the electron current density.
I. INTRODUCTION
To meet the demand for decreasing packaging size, the electron current density through the flip-chip solder joints of microprocessors has to increase with every new generation of devices. It is anticipated the current density through flip-chip solder joints will soon reach 104 A/cm2.1 At such a current density level, two concurrent processes, electromigration and Joule heating, present challenging materials problems.2 Several failure mechanisms have been reported for flip-chip solder joints under the stressing of high current density. In the void formation-and-propagation mechanism,3,4 a void first initiates at the current density crowding region. It then extends along the under bump metallurgy (UBM)/solder interface, and eventually forms a gap between the UBM and a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www. mrs.org/jmr_policy. DOI: 10.1557/JMR.2008.0024
250 J. Mater. Res., Vol. 23, No. 1, Jan 2008 http://journals.cambridge.org Downloaded: 13 Mar 2015
the solder, resulting in an open circuit failure. The main process responsible for this mechanism is electromigration. Joule heating plays a secondary role in raising the local temperature. Nevertheless, Joule heating can play a more dom
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